In recent years, high-density long-term recording of a digital image or the like has come to be performed along with the advent of high-definition television. As an optical information recording medium meeting the request, an optical information recording medium (hereinafter, an optical disk) such as a write-once hard-disk digital versatile disc (hereinafter, an HD DVD-R) or a write-once Blu-ray disc (hereinafter, a BD-R) has been developed. The optical disk has a structure having a recording layer, a reflective layer, and a protective layer formed on one principal side of an optical transparency disk substrate. In one side of the substrate on which the recording layer and reflective layer are formed, a spiral groove or concentric grooves that is called a groove is formed, and in-between adjoining grooves is formed as a convex part called a land. In such an optical disk, recording is achieved by irradiating recording laser light to the recording layer in the groove while tracking along the groove by means of an optical disk recording reproduction device, and thus forming pits (hereinafter called “marks”) so that they can be replaced with symbols. Reproduction is achieved by irradiating a laser beam for reproduction to the length nT of the mark (T denotes the length of a bit between reference channel clocks and nT denotes a length that is an n integral multiple) and length nT of a portion between such marks (hereinafter, a space) and an array of marks and spaces, thereby converting reflected light into symbols of a reproduction signal.
As for such optical disks, a high-density recording type optical disk system was constructed under the HD DVD standard or Blu-ray disc standard (hereinafter BD standard). What is important in the high-density recording technologies are requested to attain a large recording capacity and a high signal-processing speed. By meeting the requests, an unprecedented problem has arisen. That is, a problem of a signal-to-noise ratio or inter-symbol interference. The issue is attributable to the fact that when recorded pits are read by an optical head, the beam diameter of reading laser light is larger than the size of an isolated pit. What brought in order to solve the problem is a signal processing method suitable for reproduction of high-density recording information and referred to as partial response maximum likelihood (hereinafter, PRML) decoding. The PRML method is a method of a combination of a partial response (PR) characteristic, which is a reproduction technology assuming presence of inter-symbol interference as a precondition, and maximum likelihood (ML) decoding which selectively decodes the most likely signal sequence out of a reproduction signal. Further, the PRML method is to reproduce a signal using the amplitude energies of the signal at adjoining channel clock positions without performing forcible waveform equalization so as to remove inter-symbol interference. In relation to the technology of the PRML method, various arts for processing a correction quantity of a reproduction signal on the basis of an evaluation value of the reproduction signal according to a waveform in a transmission channel have been proposed in order to reduce an error occurring during decoding processing of data.
The first proposed art is a method of decreasing a symbol error rate by extending control during equalization of a reproduction signal dependent on a PR characteristic. The second proposed art is a method of decreasing the error rate by controlling a reference signal for maximum likelihood decoding. Since the present invention relates to the latter invention, typical examples will be presented below.
For example, Japanese Patent No. 3033238 describes an invention that notes a non-linear distortion which is one form of inter-symbol interference and occurs during reading of recording pits, statistically obtains a predictive sample value from a decoded signal sequence which is outputted from a maximum likelihood decoding circuit, updates the sample value according to a change in the non-linear distortion, and feeds back the control signal to the maximum likelihood decoding circuit so as to thus achieve maximum likelihood decoding.
JP-A-2005-267759 describes an invention that detects only peak levels and bottom levels of a reproduction equalization signal for a shortest symbol (3T) and a longest symbol (11T) respectively of record bits of the signal, allocates levels as Viterbi expectation values to symbols (2T to 11T) for calculation, feeds the signal to a maximum likelihood decoding circuit so as to thus perform branch metric operation.
A thesis in the IEEE 2002 “Adaptive PRML Detection in Optical Recording Media” describes a system that allows an output signal of a Viterbi detection circuit to pass through a wave divider, and feeds a signal of a reference level to a Viterbi detector using an adaptive table. The adaptive table is a reference table as sociated with each of ten binary signals.
Patent document 1: Japanese Patent No. 3033238
Patent document 2: JP-A-2005-267759
Non-patent document 1: “Adaptive Partial-Response Maximum-Likelihood Detection in Optical Recording Media” (Conference title: 2002 International Symposium on Optical Data Topical Meeting, Joint International Symposium Technical Digest (Cat. No. 02EX552), p. 269-271)